In a radio distance measuring system, radio communication is performed between a radio terminal such as a tag and a base station such as a reader, a reflected signal in which an identification code (identification ID) of the radio terminal is multiplexed is received by the base station, and the propagation time, direction, and so forth, thereof are detected. By this means, distance measurement between a radio terminal and base station, and radio terminal position measurement, are performed. With this kind of radio distance measuring system, the characteristics of a radio wave used for measurement need to be taken into consideration when configuring the system in order to improve the reliability and accuracy of measurement results.
A known distance measuring apparatus executes a communication operation, distance measurement (ranging) operation, and passive radar operation (detecting a change or movement in the radio wave propagation environment, and tracking an object) using an IEEE802.15.4a packet, as disclosed in Patent Literature 1. The transmission format used in Patent Literature 1 is shown in FIG. 1. A reader detects the position or movement of a tag in the radio wave propagation environment by using a specific preamble structure contained in an IEEE802.15.4a packet.
A sequence of packets containing one preamble is received by the reader, and the preamble in each packet is despread. Then a reference multipath profile is updated, and a “current multipath profile” of a packet being received at the present time is obtained. An object in that radio wave propagation environment is detected by comparing this “current multipath profile” with the “reference multipath profile.” Also, ground scatter—that is, a ground reflected wave that is the strongest of reflected waves in open ground without wall surfaces or the like—is eliminated, and the position, movement, and so forth of an object are detected.
Also, in Patent Literature 1, a method is disclosed whereby, in order to apply a UWB distance measuring apparatus to passive radar, a “current multipath profile” of each packet is found from a preamble in each packet following a received packet, and the “current multipath profile” is compared with a “reference multipath profile.”
Since ground scatter assumed in a conventional configuration comprises ground reflections, if a large number of signals received by a reader are accounted for by ground reflected waves, a ground reflected wave is detected by the reader as a signal with a greater amount of delay than a reflected signal from a passive tag. Also, if the heights of a reader and tag are decided, a delay profile of a specific time domain can easily be eliminated based on a geographical relationship.